1. Field of the Invention
The present invention relates to a Doppler ultrasonic diagnostic apparatus and, more particularly, to a Doppler ultrasonic diagnostic apparatus which is capable of detecting and measuring the velocity of moving reflective members and accurately displaying the movement within an organism to be examined.
2. Description of the Prior Art
Doppler pulse devices are widely used wherein an ultrasonic pulse beam is transmitted into reflective members at a fixed repetition frequency, the reflected waves from the reflective members are received, and the distance to the reflective member is measured by comparing the time difference between the transmitted signals and the received signals and at the same time the velocity of movement of the reflective members is detected and measured by detecting changes in the frequency of the received signal.
Generally, the repetition frequency of the pulse beam is selected in accordance with the distance to the reflective member. In the case of measuring reflective members within an organism which is distantly located, however, if the frequency selected is high as compared with the repetition frequency determined on the basis of the distance to the reflective members, an aliasing echo is produced which indicates that the reflective members are situated closer than the actual position, as is well known, and this makes discrimination of the distance difficult.
A similar phenomenon is seen in the case of measuring the velocity of moving reflective members. If the repetition frequency selected is low as compared with the Doppler frequency arising from the velocity of the reflective members, an aliasing echo results in a low frequency, thereby making discrimination of the velocity difficult.
In order to measure both distance and velocity without production of aliasing echo, it is known that the relationship between a maximum Doppler frequency f.sub.dmax and pulse repetition frequency f.sub.r must conform to f.sub.dmax =f.sub.r /2 in the case of a device which is capable of detecting not only absolute velocity but also whether it is positive or negative, and f.sub.dmax =f.sub.r in the case of a device which detects and measures only the absolute velocity.
In a device which is capable of determining whether velocity is positive or negative, the following relationship holds: EQU f.sub.dmax =(2V.sub.max /c).multidot.f.sub.0 =f.sub.r /2
(f.sub.0 : ultrasonic pulse beam frequency, V.sub.max maximum velocity, c: sound velocity).
From this formula, the maximum measurable velocity V.sub.max is V.sub.max =(f.sub.r /2).multidot.c/(2f.sub.0).
The maximum distance to the reflective member R.sub.max which can unambiguously be determined is given by: EQU R.sub.max =c T/2=c/(2f.sub.r)
where, T=1/f.sub.r is the pulse repetition interval.
However, as is obvious from the above formulas, such a device suffers from the problem that if the pulse repetition frequency f.sub.r is increased in order to increase the maximum measurable velocity V.sub.max, there is a decrease in the maximum distance R.sub.max at which the moving reflective members can be measured without the production of aliasing echo, thereby making it impossible to measure rapidly moving reflective members from a long distance.
Combining V.sub.max and R.sub.max gives the following relationship: EQU V.sub.max R.sub.max =c.sup.2 /(8.multidot.f.sub.0)
As is obvious from the above formula, another problem is that if a low ultrasonic beam frequency f.sub.o is selected, not only is it difficult to produce a transmission wave with a narrow pulse width but also it is impossible to form a finely focussed beam, resulting in a decrease in the distance resolution and the directional resolution, thus rendering it impossible to simultaneously establish the distance to and the velocity of a distantly located rapidly moving reflective members.